Among methods of producing acrylic acid, two-stage catalytic gas phase oxidation of propylene is most commonly and widely used in industry. This method includes a first-stage reaction for catalytic gas phase oxidation of propylene to acrolein, and a second-stage reaction for catalytic gas phase oxidation of acrolein to acrylic acid. Methods for performing these reactions which are conventionally proposed are roughly divided into the following two methods: a method of using a single reactor equipped with a plurality of reaction tubes each having a first-stage catalyst layer loaded with a catalyst suitable for the first-stage reaction (hereinafter referred to as a “first-stage catalyst”) and a second-stage catalyst layer loaded with a catalyst suitable for the second-stage reaction (hereinafter referred to as a “second-stage catalyst”); and a method of using two reactors, i.e., a first-stage reactor equipped with a plurality of reaction tubes loaded with a first-stage catalyst and a second-stage reactor equipped with a plurality of reaction tubes loaded with a second-stage catalyst.
In the former method, an inert material is additionally loaded between the first-stage catalyst and the second-stage catalyst. In general, the reaction temperature of the first-stage catalyst layer is higher than the reaction temperature of the second-stage catalyst layer. Therefore, by providing the inert material layer, acrolein-containing reaction gas can be rapidly cooled so that the temperature of the gas is reduced to a temperature range suitable for the oxidation reaction in the second-stage catalyst layer (Patent Literature 1). However, when the production of acrylic acid is performed for a long period of time, impurities (e.g., high-boiling materials, carbides, and the like which are by-products of the first-stage reaction) or the like will stick to the surface of the inert material, which leads to a problem that the reaction tube loaded with the catalysts and the inert material is blocked. If the reaction is continued with the blocked reaction tube, a pressure loss will increase, which may lead to a runaway reaction. Therefore, it is necessary to regularly extract the inert material from the fixed-bed reactor, and clean the reaction tubes when necessary. In addition, in order to extract the inert material from the reaction tubes, it is necessary to push out the inert material to which impurities or the like are firmly sticking using a rod or the like, or suck out the inert material using a suction tube or the like, and these operations disadvantageously distort and deform the inert material.
Also, typically, the inert material extracted from the reactor is mixed with the catalyst. There are known methods of separating a catalyst from an auxiliary filler (inert material) having a shape similar to the catalyst, particularly a method of separating a catalyst from an auxiliary filler made of a magnetic metal by collecting the magnetic metal auxiliary filler using a magnet (Patent Literature 2).